1. Field of the Invention
The present invention relates to a resolver comprising excitation windings and a detection winding and adopted to detect displacement of a passive member provided with the excitation windings or the detection winding.
2. Description of the Related Art
Conventionally, a resolver used for detection of a rotational angle (displacement) of a rotational member is widely known (see Japanese Patent Publication (kokoku) No. 17781/1984 and Japanese Patent Application Laid-Open (kokai) No. 144021/1987).
Such a resolver comprises two excitation windings fixedly provided at spatial positions separated by a phase difference of 90 degrees in electrical angle, and a detection winding provided on a rotation shaft. Excitation signals are input to the excitation windings, and a detection signal is output from the detection winding. Thus, the rotational angle of the rotation shaft is detected from the phase difference between one of the excitation signals and the detection signal. When the rotational angle of the rotation shaft is represented as .PHI., the phase of one excitation winding becomes sin.PHI., and the phase of the other excitation winding becomes cos.PHI.. When the excitation signals V.multidot.sin.omega.t and V.multidot.cos.omega.t having a phase difference of 90 degrees therebetween are input to the respective excitation windings, the detection signal output from the detection winding is represented by the following equation: EQU E=(V.multidot.sin.omega.t.multidot.cos.PHI.)+(V.multidot.cos.omega.t.multid ot.sin.PHI.)=V.multidot.sin(.omega.t+.PHI.).
Thus, the phase of the detection signal varies in correspondence with the rotational angle of the rotation shaft. In this way, the rotational angle .PHI. of the rotation shaft can be obtained from the phase difference between one of the excitation signals and the detection signal.
Since such a resolver requires a high degree of accuracy in detecting the rotational angle, various factors are taken into consideration in its design, such that an accurate function is established. For example, when the resolver has a bipolar configuration, each rotation of the rotation shaft coincides with a cycle of the excitation signal. Therefore, generally, a resolver having a multipolar configuration is used in order to increase angular resolution. Further, since a laminated silicon steel plate in which a plurality of winding slots are formed is used for a stator provided with excitation windings and for a rotor iron core (rotation shaft) provided with a detection winding, a uniform pitch and a uniform gap on magnetic circuits are ensured through precise machining and accurate winding technique.
However, precise machining and accurate winding technique involve not only a limit to improving detection accuracy of rotational angle, but also a considerable increase in cost.
Excitation signals of a higher frequency can reduce the number of turns in the windings. Such an angle detector having a reduced number of turns is generally known as an inductosyn, in which signals of high frequency ranging from several tens to hundreds of KHz are used in order to reduce the number of turns. However, when this technique is applied to a resolver, the high frequency makes it difficult in practice to configure an electric circuit for detection of rotational angle of the rotation shaft, from the viewpoints of accuracy, cost, and stability. Therefore, the frequency employed in a typical resolver of high precision is therefore set to several KHz.
Ultimately, since a conventional resolver requires windings each having a large number of turns and requires a high degree of accuracy in machining and winding as well, there is a limit to reduction in size and weight. Further, there is a drawback in that component (materials) costs and manufacturing costs increase. Such being the case, from a point of view of compromise, improvement in detection accuracy cannot be achieved.